1. Field of the Invention
This invention relates to radar systems, and more specifically to a short range and inexpensive radar system for use in locating reinforcing steel rods, pipes, and other nearby objects buried in concrete, soil, or behind walls and other applications.
2. Description of the Prior Art
A requirement exists for a cost-effective system capable of locating reinforcing steel rods (rebar), pipes, and other objects buried in concrete, soil, behind walls, etc. Various devices and systems currently exist to locate these various objects, but all are either limited in capability or very costly.
Zircon Corporation, Campbell, Calif., currently produces an electronic stud sensor for locating wooden and metal wall studs behind sheetrock and can sense studs up to 1" to 11/2" deep beneath other material such as sheetrock, plywood, etc.
Lawrence Livermore National Laboratory (LLNL) has developed an ultra-wideband (UWB) radar technology applicable to produce a stud-sensing type of apparatus. See U.S. Pat. No. 5,457,394 entitled "Impulse Radar Stud Finder", filed May 7, 1993, invented by Thomas Edward McEwan, incorporated by reference.
See also U.S. Patent entitled "Ultra-Wideband Motion Sensor," now U.S. Pat. No. 5,361,070; and U.S. Pat. No. 5,345,471 entitled A Differential Receiver for Ultra-Wideband Signals filed Sep. 1, 1992, both invented by Thomas Edward McEwan and also incorporated by reference. Unlike the Zircon product which relies upon sensing a change in density via capacitive loading, the above-described LLNL technology as shown in present FIG. 1 emits a sequence of ultra-wideband radar pulses without a carrier and detects deflected pulse energy caused by the transmitted pulse wavefront encountering a change in medium, i.e., air-to-metal or concrete-to-metal, hereafter referred to as the target. This reflected energy is detected and visually displayed.
The LLNL technology generates a fast pulse (typically 100 to 1000 ps wide) from a 1 MHz oscillator 10 driving a step generator 12 which is coupled to a transmit antenna 14. A separate receive antenna 16 is coupled to a sampler circuit 18 which is gated on with a delayed version of the transmit pulse by a fixed range delay generator 22 driving a second step generator 20. The receiver (including sampler circuit 18 feeding amplifier and background subtract circuit 24 coupled to display 26) thus "looks" for reflected energy at a fixed time delay after the transmit pulse has occurred. This generates a so-called "range gate", which is well known in the radar field.
This fixed range gate allows detection of objects at a fixed physical distance from the unit as dictated by a) the unit's range gate delay, b) the material(s) the pulse energy is passing through and c) the speed of light.
In a fairly fixed and repeatable situation such as sensing wall studs behind sheetrock, the range gate delay time can be fixed at a standard distance because little variation occurs in home and other building construction techniques. However, if a broad variety of sensing applications is required such as those described above, then it has been found that a fixed range gate delay unit will not suffice. The operator could have a manually variable range gate control, but this would require: a) physical scanning with the unit in both vertical and horizontal planes across a surface, b) changing the range gate control at each point desired to scan and c) doing a new background subtract (recalibration) at each of these range gate settings. This would be a very tedious and error-prone type of operation.